1.1 Fundamental Concepts - Part 1
        Introduction | Size | Material | Shape

» What is Strength of Materials
This course covers the topic known as STRENGTH OF MATERIALS. Strength of Materials is the study of how structural systems respond to loads - how they support the loads internally, and how they change shape due to those loads.

. In general, the questions that must be asked are:

1. Is the system Strong enough? e.g., can the elements carry the load without breaking or separating?

2. How does the system deflect under load? Is it Stiff enough? e.g., even if the system can support the load, does it deflect too much?
Golden Gate Bridge, San Francisco. Under a full 6-lane load of traffic, and full sidewalks, the bridge is designed to deflect 10 feet.

» Size
Strength of Materials allows you to understand how systems of a WIDE RANGE OF SIZES work. As engineers, it is important to have a good feeling of how big an object is, and how it compares relative to other systems. The rules learned in Strength of Materials apply to systems of varying size ranges: (click below to see examples)
» Shape
SHAPE is also an important consideration.

I-beam cross-sections are used in steel construction as their shape is more efficient in supporting bending loads than a rectangle.

Round cross-sections are more efficient (and much easier to analyze) than square cross-sections in supporting torsion loads in rotating shafts.

» Material
The bridge is primarily made of steel and concrete; the bicycle is made of aluminum; the AFM is made of silicon nitride. Different MATERIALS have different properties. In Strength of Materials the more important material properties include:

  • Density - mass per unit volume; weight density: weight/volume;
  • Strength - how much force per unit area a material can support;
  • Stiffness - how difficult it is to deform the material.

If the bicycle of the same shape and dimensions was instead made of steel, the qualitative response to a load will be the same, but the quantitative response will differ. E.g., steel is 3 times stiffer than aluminum, aluminum deflects three times as much; on the other hand since steel is about 3 times as dense as aluminum, the steel bike would weight three times as much.

Strength, deflection and weight are all factors. Other factors, such as resistance to corrosion and ease of manufacture may also influence the choice of one material over another. The bottom line:

» Choice of material will depend on the requirements of the application.

Updated: 07/31/2006